The present invention relates to a heavy-duty pneumatic tire, more particularly to a bead structure capable of reducing the weight of the bead portion without sacrificing the bead durability.
Recent years, in order to protect the global environment, automobiles are strongly required to reduce the fuel consumption. Accordingly, tire manufacturers devote all effort to improve tire's factors capable of reducing the fuel consumption. For example, the rolling resistance and weight of the tire are important factors. In the case of heavy-duty pneumatic tires for truck, bus and the like, in comparison with passenger car tires, the tire size is large and the tire is rigid, therefore, it is effective to improve the tire weight rather than the rolling resistance.
As well known in the art, bead portions of a heavy-duty pneumatic tires are conventionally provided with a U-shaped steel-cord reinforcing layer (c) as shown in FIG. 6 for example.
The present inventor, therefore, made a study on the bead structures in order to reduce the tire weight by reducing the weight of the steel-cord reinforcing layer.
For example, if the axially inner part and outer part of the U-shaped steel-cord reinforcing layer are reduced in the radial heights from those of the reinforcing layer as shown in FIG. 5 to those of the reinforcing layer as shown in FIG. 6, then the bending rigidity of the bead portion is decreased. Accordingly, the bead portion under load is largely bent axially outwardly, and a large stress concentrates on the radially outer end b1e of the carcass ply turnup portion b1. As a result, bead failures such as cord end loose, ply edge separation and the like becomes liable to occur, and the bead durability is decreased.